International publication WO-00/37691, incorporated herein by reference, discloses a method wherein in a first step, also called a sulfidation step, solid alumina is converted into solid aluminum sulfide (Al2S3) by reacting with gaseous carbon sulfide. In a second step, also called a separation step, the solid aluminum sulfide is fed into a separating reactor such as an electrolysis cell wherein metallic aluminum is separated from the aluminum sulfide.
This has various drawbacks. One such drawback is that it requires two separate processes: the sulfidation step and further the separation step. As a consequence the aluminum sulfide has to be transported from the reactor in which it is formed to the reactor in which the separation step is carried out. Aluminum sulfide has a very high affinity to oxygen. Therefore, any oxygen with which the aluminum sulfide comes into contact, e.g. as oxygen in air or in water, converts the aluminum sulfide back into alumina. The known process therefore puts high demands on the handling of aluminum sulphide.
Another drawback of the disclosed method is that, to perform the separation step of aluminum sulfide efficiently at a low voltage, in particular by electrolysis using inert electrodes, it is required that a large fraction, preferably all, of the alumina is converted in the sulfidation step into aluminum sulfide before the reaction components of the sulfidation step are fed into the electrolysis cell. Alumina present in an electrolysis cell operated at a low voltage is not discomposed and settles in the cell as a sludge, which has to be removed. Removal of sludge disturbs the operation of the electrolysis cell and more importantly, brings about the risk of introducing oxygen into the electrolysis cell, which converts aluminum sulfide back into alumina. Furthermore, the alumina that remains present in the electrolysis cell may dissolve and saturate the electrolyte, thus hindering further dissolution of aluminum sulfide and subsequent separation of aluminum from aluminum sulfide.
However, a nearly complete conversion of alumina into aluminum sulfide reduces the overall efficiency of the process. In practice the conversion rate slows down as the reaction proceeds, and the efficiency of the sulfidation reaction decreases, as the time, the reactor volume, and the amount of sulfidation agents required per unit of aluminum sulfide increase. A further drawback is that compounds from the separation step, in particular alumina, have to be discarded.